What happens when a substituted benzene such as chlorobenzene or methyl benzene undergoes an electrophilic substitution reaction?
There are two things to consider – is the benzene ring more or less reactive if it already has a side group and does a side group influence the position of the incoming electrophile?
If we consider our mechanism for electrophilic substitution, the slow step is the formation of the intermediate carbocation.
Substituted groups that stabilise the carbocation will make the ring more reactive to electrophiles (activating groups) and substituted groups that destabilise the carbocation will make the ring less reactive to electrophiles (deactivating groups).
Activating groups donate electrons into the benzene ring via a positive inductive or mesomeric effect, stabilising the intermediate carbocation.
Examples of activating groups include -NH2, -NHR and -NR2 (amines), -OH, -NHCOR (amides), -OCOR (esters), alkene and alkyl groups.
Deactivating groups withdraw electrons from the benzene ring via negative inductive or mesomeric effects, destabilising the carbocation.
Examples of deactivating groups include -NO2, -COOH, -COOR (esters), -CHO and COR (aldehydes and ketones), -SO3H and the halogens.
We also see that activating groups direct the incoming electrophile to position 2 or 4 on the benzene ring because these are the positions most effectively stabilised in the intermediate carbocation.
Deactivating groups direct the incoming electrophile to position 3. The exception to this rule is that if the substituted group is a halogen (Cl, Br or I) then the incoming electrophile is directed to position 2 or 4, even though the halogens are deactivating groups.
Practice questions
The following molecules were each reacted with 1-chloroethane in the presence of an anhydrous aluminium chloride catalyst.
- Determine whether each molecule will react faster or slower than benzene.
- Draw the structure of the major product(s) of each reaction.
Answers
- Faster: A, D, F ; Slower: B, C, E
